1. Technical Field
The embodiments described herein relate generally to rechargeable battery packs, and in particular to a method of reducing the equivalent series resistance (ESR) of a rechargeable battery pack, such as a Li-Ion battery pack, without sacrificing safety.
2. Description of the Related Art
Rechargeable battery packs, such as Li-Ion battery packs, are commonly used in many consumer electronics such as cell phones and personal digital assistants (PDAs). FIG. 1 is a schematic diagram of a prior art rechargeable battery pack 5, such as a Li-Ion battery pack, that may be used to provide power to a passive load 10, such as a cell phone or a PDA. In some implementations, the passive load 10 is supplanted by an active element such as a battery charger, which can recharge the rechargeable battery pack 5. As seen in FIG. 1, the rechargeable battery pack 5 includes a power cell 15, such as a Li-Ion cell, a thermal protector 20 and a protection circuit module (PCM) 25 driving the load 10. The PCM 25 includes an integrated circuit control chip 30 operatively coupled to one or more electronic switching devices 35 and 40, which in FIG. 1 are modeled as MOSFETs. As is known in the art, the PCM 25 is essentially a switch that detects abnormal currents and/or voltages and disconnects the cell 15 from the load 10, or, alternatively, a charger if the rechargeable battery pack 5 is being charged. The thermal protector 20 provides protection for the rechargeable battery pack 5 from overtemperature conditions. Overtemperature conditions can have any of several causes or combinations of causes. Overtemperature conditions can damage or impair electronic components such as those in the load 10. In the event of an overtemperature condition, it may be desirable to substantially reduce current, and thereby substantially reduce delivered power, to the load 10, thereby reducing the risk of damage or impairment. The thermal protector 20 may be, for example, a thermal fuse, a thermal breaker or a positive temperature coefficient (PTC) thermistor. Thermal protector 20 may also be either non-resettable or resettable. Non-resettable thermal protectors have lower equivalent series resistance (ESR), but once tripped, a rechargeable battery pack employing the non-resettable thermal protector is essentially no longer of any use. Resettable thermal protectors have higher ESRs, but can be tripped and reset many times.
As is known in the art, ESR is one of the main parameters determining the usable energy stored in the cell 15, and thus the usable energy stored in the rechargeable battery pack 5. Lower ESR, in general, means longer operation such as longer talk times for a cell phone. In the rechargeable battery pack 5, the ESR thereof includes the internal resistance of the cell 15, the resistance of the thermal protector 20, the resistance of the electronic switching devices 35 and 40, and the resistance of any connectors and other conductors in the circuit path to the load 10. In other words, because the thermal protector 20 is in the circuit path coupling the cell 15 to the load 10, the thermal protector 20 adds to the ESR of the rechargeable battery pack 5, and the resistance of the thermal protector 20 is not negligible. The circuit control chip 30 is not in the circuit path that includes the cell 15, the thermal protector 20 and the load 10, and does not significantly contribute to the ESR.
FIG. 2 is a schematic diagram of an equivalent conceptual circuit of the rechargeable battery pack 5 that shows each of the resistance components which add to the ESR of the rechargeable battery pack 5. In particular, the equivalent circuit includes a resistor 50 that represents the resistance of the cell 15, a resistor 55 that represents the resistance of the thermal protector 20, resistors 56 and 57 that represent the resistance of the electronic switching devices 35 and 40, and a resistor 58 that represents the resistance of the connectors and other conductors in the circuit path to the load 10 (which is represented by the resistor 59).
Since, as described above, ESR is one of the main parameters determining the usable energy in a rechargeable battery pack, it would be advantageous to be able to reduce the ESR of a rechargeable battery pack in a manner that does not adversely affect the safety of the rechargeable battery pack.